In my experience simple is best, and if you are just using the PA for vocals only, which is the best option for a small PA (electric pianos tend to have no bass end when put through them), I'd go for a small active speaker, around 150 watts, which you can plug a microphone directly into, getting rid of the need for a mixer. Also pay a bit extra for lightweight speaker stands... this comibination of lightweight and simplicity will mean the singer can setup the PA themselves. Well, you can always live in hope...
When I say speaker I mean a single speaker - for some reason people assume these things always have to be run in pairs. They don't, certainly not at the quiet volume levels you mention. I usually place the speaker behind the band and singer so we can get an idea of singer volume level relative to the band. No need to worry about feedback at quiet volumes.
The speaker I use is this one;
http://www.thomann.de/gb/the_box_pa110a.htm
Not sure if you can get it in the US thought. I've seen people with the equivalent Mackie speaker, which also seems to do a good job... I think it's this one;
http://www.thomann.de/gb/mackie_srm_350_v2.htm
Jennifer
